1. Technical Field
This invention relates to an electrode material for an electric double layer capacitor and a process for making the electrode material; an electrode for an electric double layer capacitor, made from the electrode material; and an electric double layer capacitor having the electrode.
The electric double layer capacitor having the electrode made from the electrode material according to the present invention has beneficial characteristics such as a high capacitance and a low internal resistance.
2. Background Art
A demand is rapidly expanded for an electric double layer capacitor which is compact and lightweight and exhibits high energy density, and the charge-discharge of which can be repeated. The charge-discharge of an electric double layer capacitor can be rapidly conducted, and thus, the capacitor can be utilized as a miniature electric source for memory back-up of a personal computer. Recently, it is expected to be used as a large electric source for electric automobiles from a viewpoint of preservation of environment and utilization of resources. For expansion of uses and development thereof, there is an increasing demand for further improvement of performance characteristics such as reduction of internal resistance, enhancement of capacitance and improvement of mechanical characteristics.
To improve the performance characteristics of the capacitor, various attempts are being made on a material for an electrode layer in the capacitor. The electrode material for the electric double layer capacitor comprises an electrode active material such as activated carbon as the major ingredient, and further comprises optional auxiliary ingredients such as an electrically conductive material and a binder for imparting performances such as electrical conductivity, adhesion and pliability. However, these auxiliary ingredients occasionally cause problems of reduction of electrode performances such as increase of resistance of the electrode and reduction of capacitance of the capacitor.
Japanese Unexamined Patent Publication S62-179711 discloses a process for making an electrode for an electric double layer capacitor comprising the steps of dispersing in water a powder prepared by pulverizing an activated carbon fiber; mixing together the thus-prepared aqueous carbon dispersion with latex of chlorosulfonated polyethylene as a thermoplastic binder; removing water from the thus-obtained mixture to prepare a solid; pulverizing the solid to prepare particles of an electrode material for an electric double layer capacitor; and press-molding the electrode material particles into an electrode for an electric double layer capacitor. However, the electrode material made by this process has poor processability and moldability, and a uniform electrode is difficult to make from the electrode material.
Japanese Unexamined Patent Publication H9-289142 has proposed a process for making an electrode for an electric double layer capacitor comprising the steps of dispersing activated carbon as an active material and a thermosetting binder in ketone such as acetone; spray-drying the thus-prepared liquid dispersion to prepare particles of an electrode material for an electric double layer capacitor; forming the electrode material particles into an electrode layer; and then sintering the thermoplastic binder in the particles at a high temperature, for example, at 900° C. to form the electrode for an electric double layer capacitor. However, the electrode for an electric double layer capacitor obtained by this process is rigid, and its use is occasionally limited because of poor pliability and poor mechanical strength.